Electrochemical conversion of anhydrous hydrogen halide to halogens gas using a membrane-electrode assembly or gas diffusion electrodes

ABSTRACT

The present invention relates to an electrochemical cell and a process for converting anhydrous hydrogen halide to halogen gas using a membrane-electrode assembly (MEA) or a separate membrane and electrode arrangement, such as gas diffusion electrodes with a membrane.

This application is a continuation-in-part of application Ser. No.08/644,551, filed May 10, 1996, now abandoned which is acontinuation-in-part of application Ser. No. 08/432,403, filed May 1,1995, now abandoned which in turn is a continuation-in-part ofapplication Ser. No. 08/156,196, filed Nov. 22, 1993 now U.S. Pat. No.5,411,641.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrochemical cell and a processfor converting anhydrous hydrogen halide to halogen gas using amembrane-electrode assembly or a separate membrane and electrodes, suchas gas diffusion electrodes.

2. Description of the Related Art

Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product ofmany manufacturing processes which use chlorine. For example, chlorineis used to manufacture polyvinyl chloride, isocyanates, and chlorinatedhydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as aby-product of these processes. Because supply so exceeds demand,hydrogen chloride or the acid produced often cannot be sold or used,even after careful purification. Shipment over long distances is noteconomically feasible. Discharge of the acid or chloride ions into wastewater streams is environmentally unsound. Recovery and feedback of thechlorine to the manufacturing process is the most desirable route forhandling the HCl by-product.

A number of commercial processes have been developed to convert HCl intousable chlorine gas. See, e.g., F. R. Minz,"HCl-Electrolysis--Technology for Recycling Chlorine", Bayer AG,Conference on Electrochemical Processing, Innovation & Progress,Glasgow, Scotland, UK, Apr. 21-23, 1993. The commercial processes fallinto two categories: thermal catalytic oxidation processes andelectrochemical processes.

The current commercial thermal catalytic oxidation processes forconverting anhydrous HCl and aqueous HCl into chlorine are the"Shell-Chlor", the "Kel-Chlor" and the "MT-Chlor" processes. Theseprocesses are based on the Deacon reaction. Another thermal catalyticoxidation process based on the Deacon reaction which is currently beinginvestigated, but which is not yet commercial, is the Minet process. Theoriginal Deacon reaction as developed in the 1870's made use of afluidized bed containing a copper chloride salt which acted as acatalyst. The commercial processes based on the Deacon reaction haveused other catalysts in addition to or in place of the copper used inthe original Deacon reaction, such as rare earth compounds, variousforms of nitrogen oxide and chromium oxide in order to improve the rateof conversion, to reduce the energy input and to reduce the corrosiveeffects on processing equipment produced by harsh chemical reactionconditions associated with these processes. However, in general thesethermal catalytic oxidation processes are complicated because theyrequire separating the different reaction components in order to achieveproduct purity. They also involve the production of highly corrosiveintermediates, which necessitates expensive construction materials forthe reaction systems. Moreover, these thermal catalytic oxidationprocesses are operated at elevated temperatures of 200° C. and above.

Electrochemical processes convert aqueous HCl to chlorine gas by passingdirect electrical current through a solution. The current commercialelectrochemical process is known as the Uhde process. In the Uhdeprocess, aqueous HCl solution of approximately 22% is fed at 65° to 80°C. to both compartments of an electrochemical cell, where exposure to adirect current in the cell results in an electrochemical reaction and adecrease in HCl concentration to 17% with the production of chlorine gasand hydrogen gas. The chlorine gas produced by the Uhde process is wet,usually containing about 1% to 2% water. This wet chlorine gas must thenbe further processed to produce a dry, usable gas. If the concentrationof HCl in the water becomes too low, it is possible for oxygen to begenerated from the water present in the Uhde process. Further, thepresence of water in the Uhde system limits the current densities atwhich the cells can perform to less than 500 amps./ft.² (5.38 kA/m²),because of this side reaction. The side reaction results in reducedelectrical efficiency and corrosion of the cell components.

Furthermore, electrolytic processing of aqueous HCl can be mass-transferlimited. Mass-transfer of species is very much influenced by theconcentration of the species as well as the rate of diffusion. Thediffusion coefficient and the concentration of species to be transportedare important factors which affect the rate of mass transport. In anaqueous solution, the diffusion coefficient of a species is ˜10⁻⁵ cm²/sec. In a gas, the diffusion coefficient is dramatically higher, withvalues ˜10⁻² cm² /sec. In normal industrial practice for electrolyzingaqueous hydrogen chloride, the practical concentration of hydrogenchloride or chloride ion is ˜17% to 22%, whereas the concentration ofhydrogen chloride is 100% in a gas of anhydrous hydrogen chloride. Above22% , conductance drops, and the penalty, in terms of additional power,for electrolyzing hydrogen chloride begins to climb. Below 17%, oxygencan be evolved from water, corroding the cell components, reducingelectrical efficiency, and contaminating the chlorine.

U.S. Pat. No. 4,311,568 to Balko also describes an aqueouselectrochemical process for converting HCl to chlorine. However, aqueouselectrochemical processes for converting HCl to chlorine are hampered byoxygen evolution. Oxygen evolution occurs when there is chloridestarvation in the anode, and the cell current is sustained by theelectrolysis of water derived from aqueous hydrogen chloride and/or fromwater within a hydrated membrane. Thus, in Balko, controlling andminimizing oxygen evolution is an important consideration. In general,the rate of an electrochemical process is characterized by its currentdensity. In Balko, as overall current density is increased, the rate ofoxygen evolution increases, as evidenced by the increase in theconcentration of oxygen found in the chlorine produced. Balko can run athigher current densities for a short period of time, but is limited bythe deleterious effects of oxygen evolution. If the Balko cell were tobe run at higher current densities for any length of time, the anodewould be destroyed.

Some electrochemical cells, such as that disclosed in U.S. Pat. No.4,311,568 to Balko, employ a membrane and electrodes which arephysically separate elements. Such an arrangement has non-uniformitiesin both the membrane and the electrodes, resulting in uneven contacttherebetween and less utilization of the catalyst than if the contactbetween the membrane and the electrodes were uniform. Accordingly, thecurrent density of such a cell is limited not only by the presence ofwater, as discussed above, but also by catalyst utilization. Improvedcatalyst utilization has been achieved by a membrane-electrode assembly,as disclosed in U.S. Pat. No. 5,330,860 to Grot and Banerjee. ThisPatent discloses the use of a membrane-electrode assembly in an aqueouselectrolytic cell or a fuel cell.

Thus, there exists a need to develop an electrochemical cell which isable to directly convert anhydrous hydrogen halide to essentially dryhalogen gas which can achieve much higher current densities than can beachieved by electrochemical cells of the prior art. In addition, thereexists a need to develop an electrode system which has improved catalystutilization than electrodes of the prior art.

SUMMARY OF THE INVENTION

The present invention solves the problems of the prior art by providingan electrochemical cell and a process which achieve much higher currentdensities than those achieved by electrochemical cells of the prior artby employing a membrane-electrode assembly. In particular, themembrane-electrode assembly of the present invention is characterized bya uniform coating of electrochemically active material and even particledistribution of the material.

By employing a membrane-electrode assembly, where particles ofelectrochemically active material used for an anode and a cathode areapplied directly to a membrane, the surface area contact between theelectrochemically active material and the membrane is greatly increased,as compared to separate element membrane and electrode arrangements ofthe prior art. This increased contact enables the cell of the presentinvention to be run at higher current densities at a given voltage thanelectrochemical cells of the prior art. And as current densityincreases, capital investment decreases, making the present inventionparticularly attractive from a capital investment view point.

The electrochemical cell and process of the present invention alsorequire lower operating costs than the electrochemical conversions ofhydrogen chloride of the prior art. This is because, in general, forelectrochemical conversions, as voltage increases the power cost perunit of Cl₂ produced increases. The voltage required to carry out theelectrochemical conversion of the present invention at a given currentdensity is lower than the voltage at that given current density requiredby a corresponding electrochemical conversion of the prior art,(provided that the given current density can even be achieved by theprior art). Thus, this advantage can translate directly into lower powercosts per pound of say, chlorine, generated than in the aqueouselectrochemical processes of the prior art.

Moreover, the electrochemical cell and process of the present inventionallow for direct processing of anhydrous hydrogen halide to essentiallydry halogen gas. The term "direct" means that the electrochemical cellof the present invention obviates the need to convert essentiallyanhydrous hydrogen halide to aqueous hydrogen halide beforeelectrochemical treatment or the need to remove water from the halogengas produced. This direct production of essentially dry halogen gas,when done, for example, for chlorine, is less capital intensive thanprocesses of the prior art, which require separation of water from thechlorine gas.

The electrochemical cell and process of the present invention alsoprovide a process which produces drier chlorine gas with fewerprocessing steps as compared to that produced by electrochemical orcatalytic systems of the prior art, thereby simplifying processingconditions and reducing capital costs.

To achieve the foregoing advantages, in accordance with the presentinvention, there is provided an electrochemical cell for the directproduction of essentially anhydrous hydrogen halide to essentially dryhalogen gas comprising a membrane electrode assembly. The membraneelectrode assembly includes means for oxidizing molecules of essentiallyanhydrous hydrogen halide to produce essentially dry halogen gas andprotons, cation-transporting means for transporting the protonstherethrough, wherein the oxidizing means is disposed in contact withone side of the cation-transporting means, and means for reducing thetransported protons disposed in contact with the other side of thecation-transporting means, wherein the oxidizing means and the reducingmeans comprises particles of an electrochemically active materialapplied to the cation-transporting means.

Further in accordance with the present invention, there is provided anelectrochemical cell for the direct production of essentially dryhalogen gas from essentially anhydrous hydrogen halide. Theelectrochemical cell comprises means for oxidizing molecules ofessentially anhydrous hydrogen halide to produce essentially dry halogengas and protons, proton-transporting means for transporting the protonstherethrough, wherein the oxidizing means is disposed in contact withone side of the proton-transporting means and the proton-transportingmeans is a membrane comprising a copolymer of at least two monomers, andat least one of the monomers has a pendant sulfonic acid group and meansfor reducing the transported protons, wherein the reducing means isdisposed in contact with the other side of the proton-transportingmeans.

Further in accordance with the present invention, there is provided aprocess for the direct production of essentially dry halogen gas fromessentially anhydrous hydrogen halide. The process comprises the stepsof feeding molecules of essentially anhydrous hydrogen halide to aninlet of an electrochemical cell comprising a cation-transportingmembrane, an anode disposed in contact with one side of the membrane anda cathode disposed in contact with the other side of the membrane andapplying a voltage to the anode and the cathode such that the anode isat a higher potential than the cathode, wherein the molecules of theessentially anhydrous hydrogen halide are transported to the anode, themolecules are oxidized at the anode to produce essentially dry halogengas and protons, the protons are transported through thecation-transporting membrane of the electrochemical cell, and thetransported protons are reduced at the cathode.

Further in accordance with the present invention, there is provided aprocess for the direct production of essentially dry halogen gas fromessentially anhydrous hydrogen halide. The process comprises the stepsof feeding molecules of essentially anhydrous hydrogen halide to aninlet of an electrochemical cell, wherein the cell comprises amembrane-electrode assembly including an anode disposed in contact withone side of the membrane and a cathode disposed in contact with theother side of the membrane, where the anode and the cathode compriseparticles of an electrochemically active material applied to themembrane applying a voltage to the electrochemical cell so that theanode is at a higher potential than the cathode, wherein molecules ofessentially anhydrous hydrogen halide are transported to themembrane-electrode assembly, the molecules are oxidized at the anode toproduce essentially dry halogen gas and protons, the protons aretransported through the cation-transporting membrane, and thetransported protons are reduced at the cathode.

Further in accordance with the present invention, there is provided amethod of making a membrane-electrode assembly. The method comprises thesteps of adding particles of an electrochemically active material to asolution comprising a binder polymer and a mixture ofperfluoro(methyl-di-n-butyl)amine and perfluoro(tri-n-butylamine) toform a coating formulation and coating a membrane with the coatingformulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the details of an electrochemicalcell for producing halogen gas from anhydrous hydrogen halide accordingto the present invention.

FIG. 1A is a cut-away, top cross-sectional view of the anode and cathodemass flow fields as shown in FIG. 1.

FIG. 2 is a perspective view of the electrochemical cell of the presentinvention, illustrated for the case where anhydrous hydrogen chloride isconverted to essentially dry chlorine.

FIG. 3 is a graph which plots current density vs. voltage for themembrane-electrode assembly of the present invention and for anotherelectrochemical cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention as illustrated in the accompanyingdrawings.

In accordance with the present invention, there is provided anelectrochemical cell which directly produces essentially dry halogen gasfrom essentially anhydrous hydrogen halide. Such a cell is shown at 10in FIGS. 1 and 2. In a first embodiment of the present invention, ahalogen gas, such as chlorine gas, as well as hydrogen, is produced inthe cell of the present invention. In a second embodiment, water, aswell as a halogen gas, such as chlorine gas, is produced in this cell,as will be explained more fully below.

The electrochemical cell of the present invention comprises means foroxidizing molecules of essentially anhydrous hydrogen chloride toproduce essentially dry halogen gas and protons. The oxidizing meanscomprises an electrode, or more specifically, an anode 12 as shown inFIGS. 1 and 2. The electrochemical cell of the present invention alsocomprises inlet means for introducing molecules of essentially anhydroushydrogen halide to the oxidizing means. The inlet means comprises ananode-side inlet 14 as shown in FIG. 1. The electrochemical cell of thepresent invention also includes an anode-side outlet 16 as shown inFIG. 1. Since in the illustrated case, anhydrous HCl is carried throughthe anode-side inlet, and chlorine gas is carried through the outlet, itis preferable that the inlet and the outlet are lined with afluoropolymer resin, sold by E.I. du Pont de Nemours and Company(hereinafter referred to as "DuPont") under the trademark TEFLON®(hereinafter referred to as "PFA").

The electrochemical cell of the present invention also comprisescation-transporting means, or more specifically, proton-transportingmeans, for transporting the protons therethrough, wherein the oxidizingmeans is disposed in contact with one side of the cation-transportingmeans, or proton-transporting means. Preferably, the cation-transportingmeans is a cation-transporting, or proton-transporting, membrane 18,where the anode is disposed in contact with one side of the membrane asshown in FIGS. 1 and 2.

The electrochemical cell of the present invention also comprises meansfor reducing the transported protons disposed in contact with the otherside of the cation-transporting means. The reducing means comprises anelectrode, or more specifically, a cathode 20, where cathode 20 isdisposed in contact with the other side (as opposed to the side which isin contact with the anode) of membrane 18 as illustrated in FIGS. 1 and2. Cathode 20 has a cathode-side inlet 22 and a cathode-side outlet 24as shown in FIG. 1.

Both the oxidizing means, or anode, and the reducing means, or cathode,comprise particles of an electrochemically active material. In one case,i.e., the membrane-electrode, or MEA, embodiment, the electrochemicallyactive material may applied to the cation-transporting membrane. In thiscase, the electrochemically active material forms a catalyst layer onthe membrane. The particles may be applied at or under the surface ofthe membrane. In another case, the membrane and the electrodes may beseparate elements. For example, the anode and the cathode may be porous,gas-diffusion electrodes. Such electrodes provide the advantage of highspecific surface area, as know to one skilled in the art.

In either the MEA or the separate membrane electrode case, theelectrochemically active material may comprise any type of metallicparticle or metallic oxide particle, as long as the material can supportcharge transfer and is sufficiently stable in the environment of anelectrochemical cell. The electrochemically active material may comprisea metal such as platinum, ruthenium, osmium, rhodium, iridium,palladium, titanium, and the oxides, alloys or mixtures thereof. Gold,rhenium, tin and zirconium, and the oxides, alloys or mixtures thereofmay also be used. In one embodiment, the electrochemically activematerial used for both the anode and the cathode may be rutheniumdioxide. In another embodiment, the electrochemically active material ofthe anode and the cathode may comprise platinum. It should be noted thatdifferent materials may be used for the electrochemically activematerial for the anode and the cathode. Thus, for example, in anotherembodiment, the electrochemically active material of the anode comprisesruthenium dioxide, and the electrochemically active material of thecathode comprises platinum. Other electrochemically active materialssuitable for use with the present invention may include, but are notlimited to, transition metal macro cycles in monomeric and polymericforms and transition metal oxides, including perovskites and pyrochores.

The electrochemically active material may comprise a catalyst materialon a support material. The support material may comprise particles ofcarbon and particles of polytetrafluoroethylene, (hereinafter referredto as "PTFE") which is sold under the trademark "TEFLON®", and iscommercially available from DuPont. The electrochemically activematerial may be bonded by virtue of the PTFE to a support structure ofcarbon paper or graphite cloth and hot-pressed to thecation-transporting membrane. The hydrophobic nature of the PTFE doesnot allow a film of water to form at the anode, where a water barrierwould hamper the diffusion of HCl to the reaction sites of theelectrochemically active material.

The loadings of electrochemically active material may vary based on themethod of application to the membrane. Hot-pressed, gas-diffusionelectrodes typically have loadings of 0.10 to 0.50 mg/cm². Otherloadings are possible with other available methods of deposition, suchas the MEA embodiment, where the electrochemically active material isapplied to the cation-transporting membrane. In this embodiment,loadings as low as 0.017 mg. of active material per cm² have beenachieved, although the loadings could be higher. A description of a lowloading system where electrochemically active materials are distributedonto membranes can be found in Wilson and Gottesfeld, "High PerformanceCatalyzed Membranes of Ultra-low Pt Loadings for Polymer ElectrolyteFuel Cells", Los Alamos National Laboratory, J. Electrochem. Soc., Vol.139, No. 2 L28-30, 1992.

In forming the membrane-electrode assembly of the MEA embodiment of thepresent invention, the membrane is used as a substrate for theelectrochemically active material. The membrane of the present inventionmay be made of a polymer having cation exchange groups which cantransport protons across the membrane. The cation exchange group ispreferably selected from the groups consisting of sulfonate,carboxylate, phosphonate, imide, sulfonimide and sulfonamide groups.Various known cation exchange polymers can be used including polymersand copolymers of trifluoroethylene, tetrafluoroethylene,styrene-divinyl benzene, α,β,β-trifluorstyrene, etc., in which cationexchange groups have been introduced. α,β,β-trifluorstyrene polymersuseful for the practice of the invention are disclosed in U.S. Pat. No.5,422,411. The polymer may be a hydrophobic polymer, a hydrophilicpolymer or a mixture of such polymers.

The polymer used for the membrane of the present invention may comprisea polymer backbone and recurring side chains attached to the backbonewith the side chains carrying the cation exchange groups. Preferably,the membrane comprises a copolymer of at least two fluoro or perfluoromonomers, wherein at least one of the monomers has pendant at least one,if not plural, sulfonic acid groups. For example, copolymers of a firstfluorinated vinyl monomer and a second fluorinated vinyl monomer havinga cation exchange group or a cation exchange group precursor can beused, e.g., sulfonyl fluoride groups (--SO₂ F) which can be subsequentlyhydrolyzed to sulfonic acid groups. Possible first monomers includetetrafluoroethylene, hexafluoropropylene, vinyl fluoride, vinylidinefluoride, trifluorethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), and mixtures thereof. Possible second monomers include avariety of fluorinated vinyl ethers with cation exchange groups orprecursor groups.

The polymer in accordance with the invention is an essentiallyperfluorinated polymer which preferably has a polymer backbone which ishighly fluorinated and which has ion exchange groups which are sulfonategroups. The term "sulfonate groups" is intended to refer either tosulfonic acid groups or alkali metal or ammonium salts of sulfonic acidgroups. "Highly fluorinated" means that at least 90% of the total numberof halogen and hydrogen atoms are fluorine atoms. Most preferably, thepolymer backbone is essentially perfluorinated. It is also preferablefor the side chains to be highly fluorinated and, most preferably, theside chains are essentially perfluorinated.

A class of preferred polymers for use in the present invention include ahighly fluorinated, most preferably perfluorinated, carbon backbone andthe side chain is represented by the formula --(OCF₂ CFR_(f))_(a) --OCF₂CFR'_(f) SO₃ X, wherein R_(f) and R'_(f) are independently selected fromF, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0,1 or 2, and X is H, an alkali metal, or NH₄. Polymers suitable for usewith the present invention include those disclosed in U.S. Pat. Nos.4,358,545 and 4,940,525, which have a side chain represented by theformula or O--CF₂ CF₂ SO₃ X, where X is H, an alkali metal, or NH₄.Another polymer is a perfluorocarbon backbone having a side chainrepresented by the formula --O--CF₂ CF(CF₃)--O--CF₂ CF₂ SO₃ X, where Xis H, an alkali metal, or NH₄. This polymer is available from E. I. duPont de Nemours and Company (hereinafter referred to as "DuPont") underthe trademark NAFION®, which is a copolymer of tetrafluoroethylene withvinyl ether with an SO₃ X, or sulfonic acid, precursor group. Polymersof this type are disclosed in U.S. Pat. No. 3,282,875. One particularpolymer used with the present invention is a copolymer polymerized fromtetrafluoroethylene (TFE) and a vinyl ether which is represented by theformula CF₂ ═CF--O--CF₂ CF(CF₃)--O--CF₂ CF₂ SO₂ F.

The equivalent weight of the cation exchange polymer can be varied asdesired for the particular application. For the purposes of thisapplication, equivalent weight is defined to be the weight in grams ofthe polymer in sulfonic acid form required to neutralize one gramequivalent of NaOH. In the case where the polymer comprises aperfluorocarbon backbone and the side chain is --O--CF₂--CF(CF₃)--O--CF₂ --CF₂ --SO₃ X, the equivalent weight preferably is800-1500, most preferably 900-1200. The equivalent weight of thepolymers disclosed in U.S. Pat. Nos. 4,358,545 and 4,940,525 ispreferably somewhat lower, e.g., 600-1300.

In the manufacture of membranes using polymer which has a highlyfluorinated polymer backbone and sulfonate ion exchange groups,membranes are typically formed from the polymer in its sulfonyl fluorideform since it is thermoplastic in this form, and conventional techniquesfor making films from thermoplastic polymer can be used. The sulfonylfluoride, or SO₂ F, form means that the side chain, before the membraneis hydrolyzed, has the formula --OCF₂ CF(CF₃)!_(n) --OCF₂ CF₂ SO₂ F.Alternately, the polymer may be in another thermoplastic form such as byhaving --SO₂ X groups where X is CH₃, CO₂, or a quaternary amine.Solution film casting techniques using suitable solvents for theparticular polymer can also be used if desired.

A film of the polymer in sulfonyl fluoride form can be converted to thesulfonate form (sometimes referred to as ionic form) by hydrolysis usingmethods known in the art. For example, the membrane may be hydrolyzed toconvert it to the sodium sulfonate form by immersing it in 25% by weightNaOH for about 16 hours at a temperature of about 90° C. followed byrinsing the film twice in deionized 90° C. water using about 30 to about60 minutes per rinse. Another possible method employs an aqueoussolution of 6-20% of an alkali metal hydroxide and 5-40% polar organicsolvent such as dimethyl sulfoxide with a contact time of at least 5minutes at 50°-100° C. followed by rinsing for 10 minutes. Afterhydrolyzing, the membrane can be converted if desired to another ionicform by contacting the membrane in a bath containing a 1% salt solutioncontaining the desired cation or, to the acid form, by contacting withan acid and rinsing. The membrane used in the membrane-electrodeassembly of the present invention is usually in the sulfonic acid form.

The thickness of the membrane can be varied as desired for a particularelectrochemical cell application. Typically, the thickness of themembrane is generally less than about 250 μm, preferably in the range ofabout 25 μm to about 150 μm.

The electrochemically active material is conventionally incorporated ina coating formulation, or "ink", which is applied to the membrane. Theelectrochemically active material in the form of particles having aparticle diameter in the range of 0.1 micron (μ) to 10μ. The coatingformulation, and consequently the anode and the cathode after the MEA isformed, also comprises a binder polymer for binding the particles of theelectrochemically active material together. The particles ofelectrochemically active material, when coated with the binder polymer,have a tendency to agglomerate. By grinding the particles to aparticularly small size, a better particle distribution may be obtained.Thus, in accordance with the present invention, the coating formulationis ground so that the particles have an average diameter of less than5μ, and in many cases, preferably less than 2μ. This small particle sizeis accomplished by ball milling or grinding with an Eiger mini mill,which latter technique can produce particles of 1μ or less.

The binder polymer is dissolved in a solvent. The binder polymer may bethe same polymer as that used for the membrane, as described herein, butit need not be. The binder polymer may be a variety of polymers, such aspolytetrafluoroethylene (PTFE). In a preferred embodiment, the binderpolymer is a perfluorinated sulfonic acid polymer, and the side chain ofthe binder polymer, before hydrolyzation of the binder polymer, isrepresented by the formula --OCF₂ CF(CF₃)!_(n) --OCF₂ CF₂ SO₂ F (i.e.,the SO₂ F, or sulfonyl fluoride form). The side chain, afterhydrolyzation, is represented by the formula --OCF₂ CF(CF₃)!_(n) --OCF₂CF₂ SO₃ H (i.e., the SO₃ H, sulfonic acid, or acid form). When thebinder polymer is in the sulfonyl fluoride form, the solvent can be avariety of solvents, such as FLUOROINERT FC-40, commercially availablefrom 3M of St. Paul, Minn., which is a mixture ofperfluoro(methyl-di-n-butyl)amine and perfluoro(tri-n-butylamine). Inthis embodiment, a copolymer polymerized from tetrafluoroethylene and avinyl ether which is represented by the formula CF₂ ═CF--O--CF₂CF(CF₃)--O--CF₂ CF₂ SO₂ F has been found to be a suitable binderpolymer. In addition, ruthenium dioxide has been found to be a suitablecatalyst. The sulfonyl fluoride form has been found to be compatiblewith FC-40 and to give a uniform coating of the ruthenium dioxidecatalyst on the membrane.

The viscosity of the ink can be controlled by (i) selecting particlesizes, (ii) controlling the composition of the particles ofelectrochemically active material and binder, or (iii) adjusting thesolvent content (if present). The particles of electrochemically activematerial are preferably uniformly dispersed in the polymer to assurethat a uniform and controlled depth of the catalyst layer is maintained,preferably at a high volume density with the particles ofelectrochemically active material being in contact with adjacentparticles to form a low resistance conductive path through the catalystlayer. The ratio of the particles of electrochemically active materialto the binder polymer may be in the range of about 0.5:1 to about 8:1,and in particular in the range of about 1:1 to about 5:1. The catalystlayer formed on the membrane should be porous so that it is readilypermeable to the gases/liquids which are consumed and produced in cell.The average pore diameter is preferably in the range of 0.01 to 50 μm,most preferably 0.1 to 30 μm. The porosity is generally in a range of 10to 99%, preferably 10 to 60%.

The area of the membrane to be coated with the ink may be the entirearea or only a select portion of the surface of the membrane. Thecatalyst ink may be deposited upon the surface of the membrane by anysuitable technique including spreading it with a knife or blade,brushing, pouring, metering bars, spraying and the like. If desired, thecoatings are built up to the thickness desired by repetitiveapplication. Areas upon the surface of the membrane which require noparticles of electrochemically active material can be masked, or othermeans can be taken to prevent the deposition of the particles ofelectrochemically active material upon such areas. The desired loadingof particles of electrochemically active material upon the membrane canbe predetermined, and the specific amount of particles ofelectrochemically active material can be deposited upon the surface ofthe membrane so that no excess electrochemically active material isapplied.

A particularly advantageous method of applying electrochemically activeparticles to a membrane is to use a screen printing process. It ispreferable to use a screen having a mesh number of 10 to 2400,especially mesh number of 50 to 1000 and a thickness in the range of 1to 500 μm. It is preferable to select the mesh and the thickness of thescreen and control the viscosity of the ink so as to give the thicknessof the catalyst layer ranging from 1μ to 50μ, especially 5μ to 15μ. Thescreen printing can be repeated as needed to apply the desiredthickness. Two to four passes, usually three passes, have been observedto produce the optimum performance. After each application of the ink,the solvent is preferably removed by warming the catalyst layer to about50° C. to 140° C., preferably about 75° C.

A screen mask may be used for forming a catalyst layer having a desiredsize and configuration on the surface of the cation-exchange membrane.The configuration is preferably a printed pattern matching theconfiguration of the catalyst layer. The substances for the screen andthe screen mask can be any materials having satisfactory strength suchas stainless steel, poly(ethylene terephthalate) and nylon for thescreen and epoxy resins for the screen mask.

After depositing the catalyst layer, it is preferable to fix the ink onthe surface of the membrane so that a strongly bonded catalyst layer andthe cation-transporting membrane can be obtained. The ink may be fixedupon the surface of the membrane by any one or a combination ofpressure, heat, adhesive, binder, solvent, electrostatic, and the like.A preferred method for fixing the ink upon the surface of the membraneemploys pressure, heat or by a combination of pressure and heat. Thecatalyst layer is preferably pressed onto the surface of the membrane at100° C. to 300° C., most preferably 150° C. to 280° C., under a pressureof 510 to 51,000 kPa (5 to 500 ATM), most preferably 1,015 to 10,500 kPa(10 to 100 ATM).

An alternative to printing the catalyst layer directly onto the membraneis the so-called "decal" process. In this process, the catalyst ink iscoated, painted, sprayed or screen printed onto a substrate and thesolvent is removed. The resulting "decal" is then subsequentlytransferred from the substrate to the membrane surface and bonded,typically by the application of heat and pressure.

Although the present invention describes the use of a solid polymerelectrolyte membrane, it is well within the scope of the invention touse other cation-transporting membranes which are not polymeric. Forexample, proton-conducting ceramics such as beta-alumina may be used.Beta-alumina is a class of nonstoichiometric crystalline compoundshaving the general structure Na₂ O_(x).Al₂ O₃, in which x ranges from 500(β"-alumina) to 11 (β-alumina). This material and a number of solidelectrolytes which are useful for the invention are described in theFuel Cell Handbook, A. J. Appleby and F. R. Foulkes, Van NostrandReinhold, N.Y., 1989, pages 308-312. Additional useful solid stateproton conductors, especially the cerates of strontium and barium, suchas strontium ytterbiate cerate (SrCe₀.95 Yb₀.05 O₃₋α) and bariumneodymiate cerate (BaCe₀.9 Nd₀.01 O₃₋α) are described in a final report,DOE/MC/24218-2957, Jewulski, Osif and Remick, prepared for the U.S.Department of Energy, Office of Fossil Energy, Morgantown EnergyTechnology Center by Institute of Gas Technology, Chicago, Ill.,December, 1990.

As known to one skilled in the art, if electrodes, or a catalyst layerin the case of a membrane-electrode assembly, are placed on oppositefaces of membrane, cationic charges (protons in the HCl reaction asdescribed herein) are transported through the membrane from anode tocathode, while each electrode carries out a half-cell reaction. In thepresent invention, molecules of essentially anhydrous hydrogen halideare fed to anode-side inlet 12 of electrochemical cell 10 and aretransported to the membrane-electrode assembly as described above. Themolecules of the anhydrous hydrogen halide are oxidized to produceessentially dry halogen gas and protons. A portion of the essentiallyanhydrous hydrogen halide may be unreacted and exits the cell with theessentially dry halogen gas. The protons (H+) are transported throughthe membrane and reduced at the cathode to form either hydrogen gas inthe first embodiment or water in the second embodiment, as will beexplained below. A small amount of hydrogen halide, such as hydrogenchloride, is unreacted and is transported through the membrane from theanode towards the cathode.

The electrochemical cell of the present invention further comprises ananode diffuser 23 disposed in contact with the anode and a cathodediffuser 25 disposed in contact with the cathode. The anode diffuserprovides a porous structure that allows the anhydrous hydrogen halide todiffuse through to the catalyst layer of the membrane-electrodeassembly. In addition, both the anode diffuser and the cathode diffuserdistribute current over the electrochemically active area of themembrane-electrode assembly. The diffusers are preferably made ofgraphite paper, and are typically 15-20 mil thick.

The electrochemical cell of the present invention further comprises ananode flow field 26 disposed in contact with the anode reactant diffuserand a cathode flow field 28 disposed in contact with the cathodereactant diffuser, as can be seen from FIGS. 1 and 2. The flow fieldsare electrically conductive, and act as both mass and current flowfields. More specifically, the mass flow fields may include a pluralityof anode flow channels 30 and a plurality of cathode flow channels 32 asshown in FIG. 1A, which is a cut-away, top cross-sectional view showingonly the flow fields of FIG. 1. It is within the scope of the presentinvention that the flow fields and the flow channels may have a varietyof configurations. Also, the flow fields may be made in any manner knownto one skilled in the art. Preferably, the anode and the cathode flowfields comprise porous graphite paper, and have a serpentine design asshown in particular in FIG. 2. Such flow fields are commerciallyavailable from Spectracorp, of Lawrence, Mass. The flow fields mayalternatively be made of a porous carbon in the form of a foam, cloth ormatte.

The purpose of the anode flow field and the flow channels formed thereinis to get reactants, such as anhydrous HCl in the first and secondembodiments, to the anode through anode-side inlet 14 and products, suchas essentially dry chlorine gas in the first and second embodiments fromthe anode through anode-side outlet 16. The purpose of the cathode flowfield and the flow channels formed therein is to get catholyte, such asliquid water in the first embodiment, or oxygen gas in the secondembodiment, to the cathode through cathode-side inlet 22 and products,such as hydrogen gas in the first embodiment, or water vapor (H₂ O(g))in the second embodiment, from the cathode through cathode-side outlet24. Water vapor may be needed to keep the membrane hydrated. However,water vapor may not be necessary in the second embodiment because of thewater produced by the electrochemical reaction of the oxygen (O₂) addedas discussed below.

The function and configuration of the flow fields is illustrated withrespect to FIG. 2. As shown by the dotted lines in FIG. 1, a passage 15is formed between the anode-side inlet and the cathode-side outlet, anda similar passage 17 is shown formed between the cathode-side inlet andthe anode-side outlet. These passages carry the reactants into and theproducts out of the cell through t he anode and cathode-side inlets, andthe anode and cathode-side outlets. As shown in particular in FIG. 2,anhydrous hydrogen chloride (AHCl) enters the cell on the anode-side,and essentially dry chlorine gas (Cl₂) and unreacted HCl leave the cellon the anode side. On the cathode-side water or oxygen enters the cell,and hydrogen gas or water, along with dilute HCl leaves the cell on thecathode side.

The electrochemical cell of the present invention may also comprise ananode-side gasket 34 and a cathode-side gasket 36 as shown in FIG. 1.Gaskets 34 and 36 form a seal between the interior and the exterior ofthe electrochemical cell. Preferably, these gaskets are made of theterpolymer ethylene/propylene/diene (EPDM).

The electrochemical cell of the present invention also comprises ananode current bus 38 and a cathode current bus 40 as shown in FIG. 1.The current buses conduct current to and from a voltage source (notshown). Specifically, anode current bus 38 is connected to the positiveterminal of a voltage source, and cathode current bus 40 is connected tothe negative terminal of the voltage source, so that when voltage issupplied to the cell, the anode is at a higher potential than thecathode, and current flows through all of the cell components to theright of current bus 38 as shown in FIG. 1, including current bus 40,from which it returns to the voltage source. The current buses are madeof a conductor material, such as copper.

The electrochemical cell of the present invention may further comprisean anode current distributor 42 as shown in FIGS. 1 and 2. The anodecurrent distributor collects current from the anode current bus anddistributes it to the anode by electronic conduction. The anode currentdistributor may comprise a fluoropolymer which has been loaded with aconductive material. In one embodiment, the anode current distributormay be made from polyvinylidene fluoride, sold under the trademarkKYNAR® (hereinafter referred to as "KYNAR®") by Elf Atochem NorthAmerica, Inc. Fluoropolymers, and graphite.

The electrochemical cell of the present invention may further comprise acathode current distributor 44 as shown in FIGS. 1 and 2. The cathodecurrent distributor collects current from the cathode and fordistributing current to the cathode bus by electronic conduction. Thecathode distributor also provides a barrier between the cathode currentbus and the cathode and the hydrogen halide. This is desirable becausethere is some migration of hydrogen halide through the membrane. Likethe anode current distributor, the cathode current distributor maycomprise a fluoropolymer, such as KYNAR®, which has been loaded with aconductive material, such as graphite.

The electrochemical cell of the present invention also includes ananode-side stainless steel backer plate (not shown), disposed on theoutside of the cell next to the anode current distributor, and acathode-side stainless steel backer plate (also not shown), disposed onthe outside of the cell next to the cathode current distributor. Thesesteel backer plates have bolts extending therethrough to hold thecomponents of the electrochemical cell together add mechanical stabilitythereto.

When more than one anode-cathode pair is used, such as in manufacturing,a bipolar arrangement, as familiar to one skilled in the art, ispreferred. The electrochemical cell of the present invention may be usedin a bipolar stack. To create such a bi-polar stack, anode currentdistributor 42 and every element to the right of anode currentdistributor 42 as shown in FIGS. 1 and 2, up to and including thecathode current distributor, are repeated along the length of the cell,and current buses are placed on the outside of the stack.

Further in accordance with the present invention, there is provided aprocess for directly converting essentially anhydrous hydrogen halide toessentially dry halogen gas. In operation, a voltage is applied to theanode and the cathode so that the anode is at a higher potential thanthe cathode, and current flows to the anode bus. Anode currentdistributor 40 collects current from the anode bus and distributes it,along with anode diffuser 23, to the anode by electronic conduction.Molecules of essentially anhydrous hydrogen chloride gas are fed toanode-side inlet 14 and through flow channels 30 in the anode mass flowfield 26 and are transported to the surface of anode 12. The moleculesare oxidized at the anode under the potential created by the voltagesource to produce essentially dry chlorine gas at the anode, and protons(H⁺). This reaction is given by the equation: ##EQU1## The chlorine gasexits through anode-side outlet 16 as shown in FIG. 1 and is recovered.

The protons are transported through the membrane, which acts as anelectrolyte. The transported protons are reduced at the cathode. Thisreaction is given by the equation: ##EQU2## Water is delivered to thecathode through cathode-side inlet 22 as shown in FIG. 1 and through thechannels in cathode mass flow field 28 to hydrate the membrane andthereby increase the efficiency of proton transport through themembrane. In the first embodiment, the hydrogen which is evolved at theinterface between the cathode and the membrane exits via cathode-sideoutlet 24. The hydrogen bubbles through the water and is not affected bythe electrode. Cathode current distributor 44 collects current fromcathode 20, along with cathode diffuser 25, and distributes it tocathode bus 40.

In the second embodiment, a voltage is applied to the anode and thecathode so that the anode is at a higher potential than the cathode, andcurrent flows to the anode bus. Anode current distributor 40 collectscurrent from the anode bus and distributes it, along with anode diffuser23, to the anode by electronic conduction. Molecules of essentiallyanhydrous hydrogen chloride are fed to anode-side inlet 14 and aretransported through channels of anode mass flow field 26 to the surfaceof anode 12. An oxygen-containing gas, such as oxygen (O₂ (g)), air oroxygen-enriched air (i.e., greater than 21 mol % oxygen in nitrogen) isintroduced through cathode-side inlet 22 and through the channels formedin cathode mass flow field 28. Although air is cheaper to use, cellperformance is enhanced when enriched air or oxygen is used. Thiscathode feed gas may be humidified to aid in the control of moisture inthe membrane. Molecules of the hydrogen chloride (HCl(g)) are oxidizedunder the potential created by the voltage source to produce essentiallydry chlorine gas at the anode, and protons (H⁺), as expressed inequation (1) above. The chlorine gas (Cl₂) exits through anode-sideoutlet 16 as shown in FIG. 1 and is recovered.

The protons (H⁺) are transported through the membrane, which acts as anelectrolyte. Oxygen and the transported protons are reduced at thecathode to water, which is expressed by the equation:

    1/2O.sub.2 (g)+2e.sup.- +2H.sup.+ →H.sub.2 O(g)     (3)

The water formed (H₂ O(g) in equation (3)) exits via cathode-side outlet24 as shown in FIG. 1, along with any nitrogen and unreacted oxygen. Thewater also helps to maintain hydration of the membrane, as will befurther explained below. Cathode current distributor 44 collects currentfrom cathode 20 and distributes it, along with cathode diffuser 25, tocathode bus 40.

In this second embodiment, the cathode reaction is the formation ofwater. This cathode reaction has the advantage of more favorablethermodynamics relative to H₂ production at the cathode as in the firstembodiment. This is because the overall reaction in this embodiment,which is expressed by the following equation: ##EQU3## involves asmaller free-energy change than the free-energy change for the overallreaction in the first embodiment, which is expressed by the followingequation: ##EQU4## Thus, the amount of voltage or energy required asinput to the cell is reduced in this second embodiment.

The membrane of both the first and the second embodiments must behydrated in order to have efficient proton transport. Thus, in the firstand second embodiments, the cathode-side of the membrane must be kepthydrated in order to increase the efficiency of proton transport throughthe membrane. In the first embodiment, which has a hydrogen-producingcathode, the hydration of the membrane is obtained by keeping liquidwater in contact with the cathode. The liquid water passes through theelectrodes of the membrane-electrode assembly and contacts the membrane.In the second embodiment, which has a water-producing cathode, themembrane hydration is accomplished by the production of water asexpressed by equation (3) above and by the water introduced in ahumidified oxygen-feed or air-feed stream. This keeps the conductivityof the membrane high.

For the electrochemical cell of the first embodiment, a voltage in therange of 1.0 to 2.0 volts may be applied. A current density of greaterthan 5.38 kA/m² (500 amps/ft², which is achieved in the Uhde system ofthe prior art) may be achieved at a voltage of 2 volts or less. In fact,a current density in the range of 8-16 kA/m² or greater may be achieved,with 8-12 kA/m² being the average range for current density at a voltageof 1.8 to 2.0 volts. The current efficiency, that is, the amount ofelectrical energy consumed in converting anhydrous hydrogen halide tohalogen gas, of the electrochemical cell of either the first or thesecond embodiment, is on the order of 98%-99%. In addition, in either ofthe first or second embodiments, the electrochemical cell has a utility,that is, conversion per pass, or mole fraction of anhydrous hydrogenhalide converted to essentially dry halogen gas per single pass in therange of 50%-90%, with 70% being the average. The amount of water, inthe vapor state, in the anolyte outlet due to membrane hydration is lessthan 400 parts per million (ppm), and is typically in the range of200-400 ppm.

The electrochemical cell of either embodiment of the present inventioncan be operated at higher temperatures at a given pressure thanelectrochemical cells of the prior art which convert aqueous hydrogenchloride to chlorine. This affects the kinetics of the reactions and theconductivity of the membrane. Higher temperatures result in lower cellvoltages. However, limits on temperature occur because of the propertiesof the materials used for elements of the cell. For example, theproperties of a NAFION® membrane change when the cell is operated above120° C. The properties of a polymer electrolyte membrane make itdifficult to operate a cell at temperatures approaching 150° C. Thus, arange of operating temperatures for a polymer electrolyte membrane is40° C.-120° C. However, with a membrane made of other materials, such asceramic material like beta-alumina, it is possible to operate a cell attemperatures about 200° C. Room temperature operation is possible, withthe attendant advantage of ease of use of the cell. However, operationat elevated temperatures provides the advantages of improved kineticsand increased water activity for membrane hydration. A preferred rangeof temperatures is 60° C.-90° C.

It should also be noted that one is not restricted to operate theelectrochemical cell of either the first or the second embodiment atatmospheric pressure. The cell may be run at different pressures, whichchange the transport characteristics of water or other components in thecell, including the membrane. A range of operating pressures is 30-110psig, with 60-110 preferred.

The present invention will be illustrated by the following Example.

EXAMPLE

Preparation of the Coating Formulation

The coating formulation for an MEA was prepared by adding 15 g of a 50m² /g ruthenium dioxide (RuO₂) catalyst, P-2450, commercially availablefrom Colonial Metals, Inc. of Elkton, Md., to an empty flask which hadbeen purged with dry nitrogen. After additional purging with nitrogen,200 g of a solution containing a binder polymer, 2.5 wt. % copolymerpolymerized from tetrafluoroethylene (TFE) and a vinyl ether which isrepresented by the formula CF₂ ═CF--O--CF₂ CF(CF₃)--O--CF₂ CF₂ SO₂ Fdissolved in the solvent FC-40, was added to a flask with constantstirring. The ratio of the catalyst particles to the binder polymer was3:1.

After the catalyst was fully suspended, the mixture was transferred to alaboratory ball mill and was ground overnight. After the grinding, theparticle size of the ruthenium dioxide in the mixture was, on theaverage, about 5μ, with many particles being less than 2μ. The mixturewas transferred again to a flask. Then 75-80 g of solvent were allowedto evaporate under a nitrogen purge to thicken the coating formulationand form a homogeneous mixture.

Preparation of the MEA

An unhydrolyzed membrane which was 5 mil thick and made of a copolymerpolymerized from tetrafluoroethylene (TFE) and a vinyl ether which isrepresented by the formula CF₂ ═CF--O--CF₂ CF(CF₃)--O--CF₂ CF₂ SO₂ F andhaving a nominal equivalent weight of 1100 was used as a substrate. Oneside of the membrane was screen-printed with the coating formulation toa depth of approximately 0.003", or 0.3 mils to form an MEA. The printedcoating formulation was a 2.05 cm×2.85 cm rectangle. The coatingformulation was allowed to air dry. The membrane was re weighed, thencoated again on the other side. The second coating was aligned to beprecisely opposite the first. The second coating was allowed to air dry,and then the membrane was re weighed again. On an expanded basis, (wherethe membrane was expanded as described below) the loadings were 1.04 mgRuO₂ /cm² on the one side, and 1.15 mg RuO₂ /cm² on the other side ofthe membrane.

The MEA was sandwiched between two pieces of a fluoropolymer film, soldunder the trademark TEFLON® FEP by DuPont, then heat pressed for twominutes at 120° C. at slight pressure. After cooling, the MEA wasremoved from the sandwich and placed in a standard hydrolysis bath (64%water, 20% 1-methoxy-2-propanol, 16% KOH) for two hours at 75° C. Afterremoval from the bath, the MEA was rinsed in deionized water for 20minutes, boiled in DI water for 1 hour, then briefly rinsed again.

The hydrolyzed MEA was then boiled in a 10% nitric acid solution for onehour, then removed from the acid bath and rinsed a final time in DIwater. The expanded area of the coating was 2.35"×3.2" (approximately 49cm²).

The MEA was then placed in the electrochemical cell of the presentinvention. The cell included stainless steel backer plates, copper anodeand cathode buses and a current distributor on both the anode side andthe cathode side of the cell. Each of the current distributors was madeKYNAR®-graphite. In addition, the cell included porous graphite papermass flow fields adjacent to each of the current distributors, andgraphite paper distributors between the flow fields and the membraneelectrode assembly.

Anhydrous hydrogen chloride, HCl, at a pressure of 30 psi was fed to thecell at a rate of 0.9 standard liters per minute (SLPM). The cell washeated to 70° C. and DI water was recirculated on the cathode-side ofthe membrane. The cell was operated under current control and a currentdensity of up to 13 kA/m² was obtained at less than 2 V. Thecurrent-voltage curve is shown in FIG. 3.

COMPARATIVE EXAMPLE

In this Example, a cell as described above was used, except that nodiffuser was used, and instead of a membrane-electrode assembly, aruthenium dioxide catalyst coated carbon cloth, which acted as aseparate electrode, was placed between the flow field and aperfluorinated sulfonic acid membrane on each side of the membrane. Thecatalyst-coated side of the carbon cloth was oriented toward themembrane. Anhydrous HCl at a pressure of 30 psi was fed to the cell at arate of 0.9 SLPM. The cell was heated to 80° C., and DI water wasrecirculated on the cathode-side of the membrane. The current-voltagecurve is shown in FIG. 3 also. As can be seen from FIG. 3, a givencurrent density can be obtained at a much lower voltage by using amembrane-electrode assembly according to the present invention than byusing a separate membrane and electrodes.

Additional advantages and modifications will readily occur to thoseskilled in the art. The invention, in its broader aspects, is thereforenot limited to the specific details and representative apparatus shownand described. Accordingly, departures may be made from such detailswithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A process for the direct production ofessentially dry halogen gas from essentially anhydrous hydrogen halide,comprising the steps of:(a) feeding molecules of essentially anhydroushydrogen halide to an inlet of an electrochemical cell comprising acation-transporting membrane, an anode disposed in contact with one sideof the membrane and a cathode disposed in contact with the other side ofthe membrane; and (b) applying a voltage to the anode and the cathodesuch that the anode is at a higher potential than the cathode, whereinthe molecules of the essentially anhydrous hydrogen halide aretransported to the anode, the molecules are oxidized at the anode toproduce essentially dry halogen gas and protons, the protons aretransported through the cation-transporting membrane of theelectrochemical cell, and the transported protons are reduced at thecathode.
 2. The process of claim 1, wherein the anode and the cathodecomprise particles of an electrochemically active material applied tothe membrane.
 3. The process of claim 1, further including the step ofsupplying water to the cathode-side of the membrane through an inletdisposed at the cathode-side of the membrane.
 4. The process of claim 3,further including the step of distributing the water from thecathode-side inlet through flow channels formed in a cathode mass flowfield disposed in contact with the cathode, and distributing hydrogenhalide through flow channels formed in an anode mass flow field disposedin contact with the cathode, wherein the channels in the anode mass flowfield and the channels in the cathode mass flow field are parallel toeach other.
 5. A process for the direct production of chlorine gas fromanhydrous hydrogen chloride, comprising the steps of:(a) supplyinganhydrous hydrogen chloride to an inlet of an electrochemical cellcomprising a cation-transporting membrane, an anode disposed in contactwith one side of the membrane and a cathode disposed in contact with theother side of the membrane; and (b) applying a voltage to the anode andthe cathode such that the anode is at a higher potential than thecathode, wherein the hydrogen chloride is oxidized at the anode toproduce chlorine gas and protons, the protons are transported throughthe cation-transporting membrane of the electrochemical cell, and thetransported protons are reduced at the cathode.
 6. The process of claim5, further including the step of supplying water to the cathode-side ofthe membrane through an inlet disposed at the cathode-side of themembrane.
 7. The process of claim 6, further including the step ofdistributing the water from the cathode-side inlet through flow channelsformed in a cathode mass flow field disposed in contact with thecathode, and distributing hydrogen chloride through flow channels formedin an anode mass flow field disposed in contact with the cathode,wherein the channels in the anode mass flow field and the channels inthe cathode mass flow field are parallel to each other.
 8. The processof claims 5 or 7, wherein the hydrogen chloride is anhydrous hydrogenchloride which is oxidized at the anode.
 9. The process of claims 5 or7, wherein the voltage is in the range of 1.0 to 2.0 volts.
 10. Theprocess of claims 5 or 7, wherein the current density is greater thanabout 5.38 kA/m² at a voltage of 1.8 to 2.0 volts.
 11. The process ofclaim 10, wherein the current density is in the range of 8-12 kA/m² at avoltage of 1.8 to 2.0 volts.
 12. The process of claims 5 or 7, whereinthe temperature of the electrochemical cell is in the range of 40°-120°C.
 13. The process of claim 12, wherein the temperature of theelectrochemical cell is in the range of 60°-90° C.
 14. The process ofclaims 5 or 7, wherein the pressure of the electrochemical cell is inthe range of 30-110 psig.
 15. The process of claim 14, wherein thepressure of the electrochemical cell is in the range of 60-110 psig. 16.The process of claim 5 or 7, wherein the anode and the cathode compriseparticles of an electrochemically active material applied to themembrane.
 17. The process of claims 1 or 16, wherein the voltage is inthe range of 1.0 to 2.0 volts.
 18. The process of claims 1 or 16,wherein the current density is greater than about 5.38 kA/m² at avoltage of 1.8 to 2.0 volts.
 19. The process of claim 18, wherein thecurrent density is in the range of 8-12 kA/m² at a voltage of 1.8 to 2.0volts.
 20. The process of claims 1 or 16, wherein the temperature of theelectrochemical cell is in the range of 40°-120° C.
 21. The process ofclaim 20, wherein the temperature of the electrochemical cell is in therange of 60°-90° C.
 22. The process of claims 1 or 16, wherein thepressure of the electrochemical cell is in the range of 30-110 psig. 23.The process of claim 22 wherein the pressure of the electrochemical cellis in the range of 60-110 psig.
 24. The process of claims 1 or 16,wherein the anhydrous hydrogen halide is hydrogen chloride.
 25. Theprocess of claims 1 or 16, further comprising the step of contacting themembrane with water on the cathode-side of the membrane.
 26. The processof claims 1 or 16, further comprising the step of contacting themembrane with an oxygen-containing gas on the cathode-side of themembrane.